Selective cloaking circuit for use in radio frequency identification and method of cloaking RFID tags

ABSTRACT

The output to the antenna of an RFID tag or label is disconnected from the balance of the RFID chip by means of a series switch activated in response to a logic command, CLOAK, generated by the RFID chip. Activation of the switch disconnects the output to the antenna of the RFID tag without disconnecting its input. The output to the antenna is thus disconnected for a time sufficient to allow the remaining RFID tags in an RF interrogation field to be identified. However, the input of the antenna remains connected and is capable of receiving at any time, including during the cloaking period commands which can lift the tag out of cloaking and allow it to then selectively output its signal.

This application claims priority as a continuation application under 35U.S.C. §120 to U.S. Utility patent application Ser. No. 09/768,327entitled, “A SELECTIVE CLOAKING CIRCUIT FOR USE IN A RADIOFREQUENCYIDENTIFICATION AND METHOD OF CLOAKING RFID TAGS”, filed on Jan. 23,2001, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to radio frequency identification tags or labelsand in particular to a cloaking circuit used to assist in the readoperations of RFID transponders.

2. Description of the Prior Art

In U.S. Pat. No. 5,963,144, incorporated herein by reference, what isdescribed is an antenna of an RFID tag or label which is disconnectedfrom the balance of the RFID chip by means of a series switch activatedin response to a logic command, CLOAK, generated by the RFID chip.Activation of the switch disconnects the antenna of the RFID tag fromthe remainder of the RFID chip and effects a high impedance resistanceacross the antenna terminals. An RC circuit is charged by activation ofthe CLOAK signal and thereafter discharges during a predetermined RCtime period as determined by a high impedance series antifuse leakagetransistor. The antenna is thus disconnected for a time sufficient toallow the remaining RFID tags in an RF interrogation field to beidentified. Meanwhile, during the disconnection of the antenna from theRFID chip and its loading causes its effective absorption and scatteringaperture to be reduced to near zero so as to electromagnetically removethe RFID tag from the zone of interrogation during the predeterminedtime period. Hence, the interrogated tag remains disconnected andnoninterferring with the RF field used to interrogate the remainingtags. It is also known to detune the tag's antenna by at least partiallyshorting out the antenna, as either a means of signaling or fordepleting the energy stored in the antenna resonant structure.

Thus what is taught is disconnecting the front end of the RFID chip evenduring times when the power to the tag has been removed. This could beaccomplished by open circuiting the data path and/or the power input.Unfortunately, this means that while the chip is in the Cloak state, itis impossible for the reader to communicate with the tag.

As an example, a tag with a cloak time of 20 seconds might beinterrogated and then cloaked while on a conveyor belt. But it might benecessary to read the label subsequently when the tag has traveledfurther down the conveyor belt but within the 20 second time period.

What is needed is a circuit and method that allows a tag to be cloaked,but still to be interrogated when it is cloaked.

BRIEF SUMMARY OF THE INVENTION

The invention is defined as an improvement in a cloaked RFID tag havingan antenna comprising a switch and a logic circuit coupled to theswitch. The logic circuit or gate selectively allows communication ofsignals through the antenna during normal operation to thereby allowoutput of a signal from the RFID tag through the antenna and to disablethe RFID output during a cloaking period. A receiving connection isprovided to the RFID tag so that command signals are continuouslyreceivable notwithstanding cloaking of the RFID tag.

The RFID tag includes an input circuit. The receiving connection is anelectrical connection between the antenna and the input circuit which isnot interrupted by operation of the switch. The electrical connectioncomprises a diode coupled between the antenna and the input circuit. Theswitch is a grounding switch, such as a switching transistor, coupledbetween the antenna and ground. The switch communicates signals throughthe antenna by selectively grounding the antenna according to thesignals during the normal operation. The logic circuit couples signalsto the switch to ground the antenna during the normal operation andisolates signals from the switch during the cloaking operation. Furtherpower is supplied through the antenna to the RFID circuit during thecloaking operation.

The invention further comprises a method for performing the foregoingoperations.

The invention now having been summarized, turn to the following drawingin which like elements are referenced by like numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified circuit diagram of the illustrated embodimentwhere the RFID tag remains in a condition in which command signals canbe received during the cloaking period and in response to which the tagmay be awakened on command.

FIG. 2 is a timing diagram of the operation of the circuit of FIG. 1.

The invention now having been visualized in the foregoing drawings, theinvention and its various embodiments may now be better understood byturning to the following detailed description of preferred embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the present invention the chip's output is disabled so that the chipcannot respond even though it is receiving information from the reader.The advantage of such a scheme is that a command can be introduced suchthat the Cloak state could be overcome allowing the tag 10 to respondduring the Cloak period. The normal command waking the chip is stillused such that any tag 10 in the Cloak state would still not bedetected.

Before discussing this improvement, first consider some foundationalbackground information concerning the operation of a cloaked RFID tag 10in general. Cloaking a circuit or RFID tag 10 effectively disconnectsthe tag's antenna 42 from the rest of the tag 10. This effectivedisconnection is done by means of circuits on the RFID tag 10 that aredesigned to: (1) form a series switch between the antenna terminals andthe logic circuitry of a chip comprising the tag 10; and (2) provide ameais to maintain the switch in an open or antenna-disconnected statefor a time period significantly long enough relative to the timerequired to identify remaining tags in the RF field. The time periodsare determined by the voltage decay of a resistor-capacitor circuitwhich continues to perform its function even when the tag 10 power isremoved. In the illustrated embodiment, an anti-fuse structure is usedas a high-valued resistor to achieve these long RC time cost delays.Without the improvement to the invention disclosed below the antenna 42of the RFID tag 10 remains disconnected or loaded by a high impedanceduring the power on/off cycles, in the absence of the powering RF fieldand until the RC delay times out. The time delays are dependent on theexact process parameters utilized in fabricating the RFID tag 10 andtypically may be in the range of 2-100 seconds.

During this time-out period, the impedance coupled to the antenna 42terminals by the RFID chip is raised sufficiently high so that theantenna 42 appears to be effectively open-circuited. This causes theantenna 42 and the RFID tag 10 to have both reduced absorption andreflection of the RF energy. This in turn amounts to a novel applicationof antenna theory, where an antenna 42 whose terminals areopen-circuited, both have a minimum effective absorption and scatteringaperture for the RF interrogation and power field.

The advantage of the performance of an RFID system of the invention isthat during the time that the antenna 42 is effectively disconnected,the tag 10 appears less visible in the RF field or is cloaked. Thecloaked antenna 42 interferes less with other tags, which are then goingthrough the interrogation or identification process. Further, as eachtag 10 is in turn identified, a coded signal is transmitted to tell theidentified tag 10 to cloak itself. When in the cloaked state, thisallows therefore more energy in the RF field to be available for readingthe remaining tags in the interrogation zone. The overall capacity toread and identify multiple tags within the read range of a RFID systemis significantly improved as a result of applying this concept.

Turn now and consider the improvement of the cloaking system describedabove as shown in FIG. 1 by circuit 10 which is included in an RFID tag.In normal operation (cloak not activated), the “Cloak bar” node 12coupled to the input of AND gate 16 is high. Whenever “Output” on node14 goes high (typically for 1 to 2 psec) input pad 18 to which the tagantenna is connected is shorted. In other words with both Output node 14and Cloak bar node 12 high, AND gate 16 has a high output coupled to thegate of transistor 20, which then couples pad 18 through diode to groundpad 22. This causes a backscattered signal from the grounded tag antenna42 coupled to pad 18 to be produced and then detected by the RFIDreader.

FIG. 2 is a timing diagram of the relevant signals in which the envelopeof the input signal is shown on line 32, the Output at node 14 on line34 and Cloak baron line 36. During normal operation 28 when Output goeshigh at node 14 as shown by pulse 38, a backscattered pulse 40 will bepresented on pad 18, i.e. a momentary grounding of the antenna 42.However, during a cloaked period 30, Output pulse 38 is cut off from theantenna 42. In the cloak mode (cloak activated), the “Cloak bar” node 12is low. Therefore, the output of the AND gate 16 remains low at alltimes and no signal can be backscattered from the RFID tag 10.

However, as shown in FIG. 1 input pad 18 is connected at all timesthrough diode 24 to the input stages 44 of the RFID tag 10 and powercontinues to be supplied to the tag 10 through diode 26 to the tag powercircuitry 46. Thus, commands may thus be sensed, read and processed byRFID tag 10 even when the output of the tag 10 is cloaked. With theimprovement the tag 10 can now be brought out of out a cloaked state forsubsequent interrogations by the reader. A command can be transmitted,sensed and processed at any time through the antenna 42, input pad 18,and to the tag input circuitry 44 through diode 24 to bring Cloak Barhigh and allow the antenna signal to be controlled by Output at node 14.

Many alterations and modifications may be made by those having ordinaryskill in the art without departing from the spirit and scope of theinvention. Therefore, it must be understood that the illustratedembodiment has been set forth only for the purposes of example and thatit should not be taken as limiting the invention as defined by thefollowing claims.

The words used in this specification to describe the invention and itsvarious embodiments are to be understood not only in the sense of theircommonly defined meanings, but to include by special definition in thisspecification structure, material or acts beyond the scope of thecommonly defined meanings. Thus if an element can be understood in thecontext of this specification as including more than one meaning, thenits use in a claim must be understood as being generic to all possiblemeanings supported by the specification and by the word itself.

The definitions of the words or elements of the following claims are,therefore, defined in this specification to include not only thecombination of elements which are literally set forth, but allequivalent structure, material or acts for performing substantially thesame function in substantially the same way to obtain substantially thesame result. In this sense it is therefore contemplated that anequivalent substitution of two or more elements may be made for any oneof the elements in the claims below or that a single element may besubstituted for two or more elements in a claim.

Insubstantial changes from the claimed subject matter as viewed by aperson with ordinary skill in the art, now known or later devised, areexpressly contemplated as being equivalently within the scope of theclaims. Therefore, obvious substitutions now or later known to one withordinary skill in the art are defined to be within the scope of thedefined elements.

The claims are thus to be understood to include what is specificallyillustrated and described above, what is conceptionally equivalent, whatcan be obviously substituted and also what essentially incorporates theessential idea of the invention.

1. (canceled)
 2. A method of operating an RFID tag comprising: disablingan output of an RFID tag during a cloaking operation; receiving inputtransmissions regardless of the cloaking operation; enabling the outputof the RFID tag during a cloaking operation responsive to a receivedinput transmission.
 3. The method of claim 2, wherein the disabling stepcomprises causing a switch to open.
 4. The method of claim 2, whereinthe disabling step comprises causing a transistor to turn off.
 5. Themethod of claim 2, wherein the disabling step comprises disconnecting anantenna of the RFID tag from ground.
 6. The method of claim 2, whereinthe disabling step comprises disabling backscattering from the RFID tag.7. The method of claim 2, wherein the receiving step comprisesconnecting an antenna of the RFID tag to a receive portion of the RFIDtag regardless of the cloaking operation.
 8. The method of claim 7,wherein the connection between an antenna of the RFID tag and a receiveportion of the RFID tag is continuous.
 9. The method of claim 2, whereinthe enabling step comprises causing a switch to close during a cloakingoperation.
 10. The method of claim 2, wherein the enabling stepcomprises causing a transistor to turn on during a cloaking operation.11. The method of claim 2, wherein the enabling step comprisesconnecting an antenna of the RFID tag to ground during a cloakingoperation.
 12. The method of claim 2, wherein the enabling stepcomprises enabling backscattering from the RFID tag.
 13. The method ofclaim 2, further comprising supplying power through an antenna of theRFID tag during a cloaking operation.
 14. The method of claim 2, furthercomprising receiving command signals through an antenna of the RFID tag.15. The method of claim 14, wherein the command signals can be receivedduring a cloaking operation.
 16. The method of claim 2, furthercomprising communicating data signals through an antenna of the RFID tagby grounding the antenna so as to cause backscattering of a receivedsignal.
 17. The method of claim 16, wherein the communicating of datasignals occurs when uncloaked.
 18. The method of claim 16, wherein thecommunicating of data signals occurs when cloaked responsive to acommand to the RFID tag.